Have you noticed (probably yes) that Atton is the complete opposite of Carth in terms of their back stories.
Both served against the Mandalorians. Carth represented (in terms of story) the soldiers that remained loyal to the Republic - as Atton represented the Soldiers, that became Sith. He showed the other side to Carth's story.
I thought that was pretty clever story writing.

This is a very good observation, and I never really thought about it before you mentioned it. I wonder if perhaps it was in response to the small cult of people calling themselves "Carth Haters". I wasn't one of them, but I can see where they'd want to give you the complete opposite of what you got for a companion in KOTOR 1. Also, Atton has the Han Solo thing going on. He may be a scoundrel, but he's capable of making the right decisions at times (if your influence swings him in that direction). Carth studiously avoids even small interactions with you, but Atton starts hitting on you from the minute he lays eyes on you if you're female.